452 validating sender: 4.3.1 insufficient system resources

452 4.3.1 Insufficient system resources
Not a good thing the night before a presentation – Murphy’s law at work again!

The Application Event Log has Event ID 15002 from MSExchangeTransport saying “The resource pressure is constant at High. Statistics… “. The event goes on to tell you that inbound mail submission has stopped, and it’s due to disk space being low on the volume where the queue database is located.

Figure 1: Event ID 15002 logged by MSExchangeTransport

Exchange Server 2007 transport queues are not the familiar .eml files you see in Exchange Server 2003/2000, which reside in the \mailroot\vsi <1>\queue folder (<1> is the instance number of the SMTP virtual server) on the file system. Yes, transport queues have been moved to a JET database of its own.

What’s Back Pressure?

In Exchange Server 2007, the Transport service monitors system resources such as disk space and memory on Transport servers (the Hub Transport and the Edge Transport servers), and stops message submission if it’s running low on these resources. It continues to deliver existing messages in the queue. When resource utilization returns to normal, it resumes message submission. The feature is called Back Pressure.

In this case, Exchange required 4 Gigs of free disk space on the volume where the Queue database was located – I had about 3.95 Gigs. 🙂

Changes to Back Pressure settings in Exchange Server 2007 SP1

The Back Pressure settings in Exchange Server 2007 RTM stop inbound mailflow if free disk space is below 4 Gigs. This static threshold has been lowered in SP1 to a more realistic 500 MB.

The Resolution

Many configuration options for transport servers are saved in an XML file named EdgeTransport.exe.config (it’s the same file name on both server roles— Edge Transport and Hub Transport) located in \Exchange Server\Bin\.

To get transport to resume submissions, you can use any of the following methods. All of the following require you to edit the EdgeTransport.exe.config file.
1.Disable BackPressure: Although Microsoft doesn’t recommend it, it does provide a way to Disable Back Pressure
2.Tweak BackPressure thresholds: Modify BackPressure parameters to more accurately define what’s high utilization for your deployment or server configurations, as explained in the above docs.
3.Move the queue database to another volume: Another resolution, and the one I used in this case, was to move the queue database to another volume with ample of free space, using the following procedure: 1.
Add the following key in the section in EdgeTransport.exe.config, as documented in “How to Change the Location of the Queue Database“:

2.Save the file and restart the Microsoft Exchange Transport service from the Services console or by using the Restart-Service cmdlet (Restart-Service MSExchangeTransport).